Inhalant therapy: Finding its place in small-animal practice
The use of inhaled respiratory medications in dogs and cats is becoming more common. Inhalant delivery of aerosolized medication offers a number of theoretical benefits, including an enormous absorptive surface area across a permeable membrane, a low enzyme environment that results in little drug degradation, avoidance of hepatic first-pass metabolism, and reproducible absorption kinetics. When the target of inhaled medications is the respiratory tract itself, additional benefits include the potential for attaining a high drug concentration directly at the disease site with minimal systemic absorption and toxicity. Often, therapeutic effect can be achieved with only a fraction of the dose required for systemic delivery of the same drug.1
Because of these advantages, inhalant delivery of medication has gained widespread use for treating airway diseases in people. More than 30 drugs licensed for people are available for inhalation, including anti-inflammatory drugs and bronchodilators. An enormous body of evidence in the medical literature exists regarding the efficacy and toxicity of inhalational drug therapy in people. In veterinary medicine, the literature on inhalant therapy to treat naturally occurring disease is sparse. Regardless, aerosol delivery of medication has become popular for treating dogs and, especially, cats with respiratory disease.
CAUTIONSAlthough inhalant drug delivery has many benefits, difficulties in using this route exist as well. Respiratory defenses are efficient at preventing particulates from reaching the lower airways, so it should come as no surprise that only a small proportion of the administered medication reaches the lower airways; a marked amount of drug is lost in the delivery device or deposited in the oropharynx.1 In animals, another portion will be deposited on the fur, especially if an animal is placed in a tent or tank, and can be ingested through grooming.
Another difficulty is that most aerosol drug delivery devices are designed to be used by people on a voluntary basis, and some require purposeful respiration and breath-holding. Adaptations of some devices facilitate their use in animals, and modified systems are now marketed for dogs and cats (e.g. Aerokat—Trudell Medical International; NebulAir Small Animal Chamber—DVM Pharmaceuticals; Breathe Easy—Jorgensen Laboratories).
Drug delivery by the aerosol route depends in part on respiratory depth and rate, tidal volume, and airflow rate, yet all of these may be negatively affected by respiratory disease. Additionally, not all drugs are suitable for aerosol delivery, and the drugs themselves (or preservatives contained in the drug preparation) may cause airway irritation and possible bronchoconstriction that could worsen respiratory function.
AEROSOL DELIVERY SYSTEMS
Two basic types of aerosol delivery systems are used in veterinary practice: nebulizers and pressurized metered dose inhalers (MDIs). The two are distinct devices with different uses. In general, nebulizers deliver much smaller particles, allowing deeper respiratory penetration, and provide fluid along with the drug.1 MDI devices deliver drugs primarily to the larger airways.
Basic nebulizer types include jet nebulizers and electronic nebulizers. Modifications exist (e.g. spinning disk nebulizers, vibrating mesh nebulizers) to improve delivery or modulate particle size.
How they work. Jet nebulizers have a compressed air or oxygen source, a well into which fluid or a drug can be placed, and a baffle that when hit by the drug creates small particles. Jet nebulizers tend to be larger and sturdier than electronic nebulizers. Electronic nebulizers use membrane vibration to produce an aerosol and are much smaller since no air compressor is required. However, in my and other clinicians' experience, electronic nebulizers tend to malfunction more easily than jet nebulizers do.